Plastics are used in various industrial fields, and particularly in durable goods, such as electronic equipment and automobiles, an improvement in strength is required.
For related art for improving the strength of plastics, metal oxides (silica, alumina, titania, and the like) in the form of fine particles are added as fillers. In recent years, attention has been paid to the high reinforcement effect of the addition of silica ultrafine particles having a diameter of 1 micron or less, particularly nanosize.
However, silica fine particles aggregate easily because of the hydroxyl groups on the surface, and therefore, the dispersibility in a plastic is low. In addition, the adhesion at the plastic/fine particle interface may be insufficient, depending on the type of a plastic, and frequently, a sufficient improvement effect is not obtained.
Accordingly, the dispersibility of silica fine particles in a plastic, and the adhesion at the plastic/fine particle interface are improved by treating the surface of the silica fine particles with reactive organic compounds, for example, organometallic compounds, such as organic silicon compounds, organic aluminum compounds, and organic titanium compounds, and various carboxylic acids. For example, Patent Document 1 (JP2004-269773A) describes that by sequentially surface-treating an oxide compound, such as silica, using a modifier including a hydrophobic group, and a modifier including a polar group, and adding the oxide compound to a thermoplastic resin, a resin composition in which the oxide compound is uniformly dispersed and firmly bonded to a functional group of the thermoplastic resin is obtained. In addition, Patent Document 2 (JP62-236821A) describes an epoxy resin composition for sealing a semiconductor, including a mixture of an epoxy resin, a curing agent, and an inorganic filler in which the surface of an inorganic substance, such as silica, is sequentially coated with a silicone rubber layer and a layer reactive with the resin.
For another surface treatment technique, higher functionality is studied, for example, the stress in the surface layer of silica fine particles is relaxed by providing a specific organic compound layer, for example, a polysiloxane layer, on the surface of the silica fine particles. As an example of this, Patent Document 3 (JP2002-327030A) describes a core-shell type fine particle including a silica fine particle as a core, and a shell layer including a polyorganosiloxane and a polymer of a hydrophobic polymerizable monomer provided on the surface of the silica fine particle via a covalent bond.
On the other hand, in recent years, the so-called sol-gel nanocomposites have been studied in which an alkoxysilane compound is previously mixed in a plastic, and then, the alkoxy groups are condensed to form nanosized silica fine particles or fine particles having a structure similar to that of the nanosized silica fine particles (hereinafter, these are collectively referred to as “silica-based fine particles”) in the plastic. This method is effective for higher dispersion of silica-based fine particles in a plastic, and as a result, a high reinforcement effect, particularly an improvement in rigidity (elastic modulus, Young's modulus), can be promoted. As an example of this, for example, Patent Document 4 (JP6-136321A) describes that a polyurethane-silica hybrid is manufactured by dissolving polyurethane, hydrolyzable alkoxysilane (or a partial condensate thereof), and a catalyst, as required, in a lower alcohol, applying the obtained alcohol sol solution to a substrate, and drying the alcohol sol solution. In addition, Patent Document 5 (JP2006-183021A) describes that a polyurethane-polysilicic acid composite is obtained by reacting water-dispersible polyurethane having a hydrophilic group, silicate, and an acid, or reacting this water-dispersible polyurethane and alkylsilicic acid under a neutral or alkaline condition to simultaneously precipitate polyurethane and polysilicic acid.